In the field of looms of the so-called "Jacquard" type, it is well known that there is a problem connected with the deposit of dust onto the loom base, and in particular the deposit of such dust close to the harness cord return springs or elastic elements. In fact environment dust and the powders and downs produced by the fibers being worked descend along the harness cords and are deposited upon the base of the loom adjacent the return springs, thus interferring with the correct raising and lowering movements of the harness cords and their springs or lowering elements and therefore the warp threads. In order to avoid the loom jamming, a periodic cleaning is necessary, which cleaning is somewhat complicated taking also into account the fragility of the area to be cleaned. In addition, in case of looms having return springs on the harness, the reciprocating extension movement of the springs causes dust to be compacted therebetween forming at each spring a pad that first slows down the return function of the springs and finally inhibits their operation. Removal of these dust pads is a long and delicate operation imposing high and often unacceptable down times. In order to obviate the above problems, in the known art compressed air devices have been proposed which are generally embodied as carriages sliding sideways of the harness so as to direct air jets against said harness, perpendicularly to the cord extension. The side positioning of the jets has been universally chosen due to the belief that the air jets transversely passing through the harness cords should be capable of blowing dust away towards the outside, thereby preventing it from accumulating at the loom base.
Although this cleaning method is somewhat efficient, it suffers from a great number of drawbacks. In fact, the air jet pushes dust transversely through the thick and dense barrier consisting of the great number of parallel springs which substantially act as a sieve, thereby trapping a certain amount of dust in the innermost harness portion. In addition, due to the resistance offered by the springs or elastic elements to be passed through, the efficiency of the air jet decreases as said jet penetrates into the return elements. As a result, an important lack of homogeneity occurs in the cleaning action, so that cleaning is excellent in the lowering elements closer to the compressed air delivery nozzles, but is greatly insufficient in the farthermost elements which not only are not conveniently cleaned from the dust coming from top, but are also impinged on by the dust entrained by the air blow.
In the known art attempts have been made to obviate the above problem by increasing the power of the air jets, and actually cleaning in general is improved. However an undesirable side effect has been found in this case, i.e. bending of the lowering elements in the harness submitted to the air pressure occurs, which bending impairs the perfect operation of the loom. Attempts have been also made to create systems having more diffuse air streams for example by means of perforated ducts or chambers into which air is blown. Said air coming out of the holes then impinges on all the return elements. However these systems have a great air consumption and low efficiency.